The colour change is normal and will happen over time with use of your pizza oven. For functionality, the oven door and chimney stack are not insulated, but continued use will.
The accessory brush is perfect to clear any embers and ashes from your stones. You do not need to use water or soap to the interior of the pizza oven to clean it.
If you notice your cooking stones begin to change colour, heat the oven to 650°F degrees, and let it cool. The cooking stones will return to normal colouring.
Once your oven has reached cool down phase, it is recommended to use a stainless steel cleaner to return the oven to its original shine.